OPTIC LOBES 77
merely selected cases, which do not indicate a wide applicability
of such a g3neralisation. Thus it is true that the brain of a Man
is more elaborate in its furrows and convolutions than is that of
a Cat. The real fact of the matter is, that the complexity of the
brain from this point of view increases with the size of the animal
within the group.
The Gorilla and the Chimpanzee have a more furrowed brain
than has the little Marmoset; the Bear a more complicated
brain than the Weasel, etc. The most highly-convoluted brains
of all mammals are those of the Elephants, and there does not
A, Mjp.^ °'hl
FlO. 50.—Lepus cuniculus. Longitudinal vertical section of tlie brain. (Nat. size.)
«.co, Anterior commissure ; &Jfo, body of the fornix ; cb) cerebellum, showing arbor
vitae ; c.c, crus cerebri ; c.h\ parencephalon or cerebral hemisphere ; c.h2, temporal
lobe ; c.m<z, corpus mammillare ; cp.d, corpus callosum ; fiwb, foramen of Monro ;
inf, infundibulum ; l.t, lamina terminalis ; Iy9 lyra ; m.co, middle commissure ;
m.o, medulla oblongata ; o.cfa, optic chiasma ; o.l\ o.Z2, corpora quadrigemina or
optic lobes ; olf, olfactory lobe ; p.co, posterior commissure ; pd.pn, peduncle of the
pineal '* gland," pn • p.fo, anterior pillar of the fornix ; pty, pituitary body ; pvM,
pons Varolii ; sp.lut septum Iticidtim j -y4, foxirth ventricle j vl.ip, velum interposi-
tum ; v.vn, valve of Vieussens ; II9 optic nerve. (From Parker's Zootcmny.}
seem in the Ungulates to be so marked a relation between size
and abundance of fissures as there is among other mammals.
A. regular plan of the fissures can be detected with certainty
for each group considered by itself; but it is not so easy to
homologise the details of arrangement from group to group.
This is so far in accord with the view that the existing groups
of mammals have diverged from each other ab initio.
Another marked characteristic of the mammalian as opposed
to other brains is the relatively small importance in size and yet
the fourfold nature of the optic lobes. "What was the case with
the optic lobes of the early Ungulates is difficult to understand,
on account of the fact that the casts are necessarily imperfect.